I. Field of the Invention
This invention relates to a third-order nonlinear optical element. The optical element of the present invention can be used in high speed optical data processing systems and optical communication systems.
II. Description of the Related Art
When a strong electric field (E) such as laser beam is applied to a substance, the substance shows electric-polarization (P) which is expressed by the following general equation: EQU P=X.sup.(1) E+X.sup.(2) EE+X.sup.(3) EEE+
(wherein X(1) means linear optical susceptibility and X.sup.(i) (i is an integer of not less than 2) means nonlinear optical susceptibility)
The nonlinear optical effects are those expressed by the high order terms of E, i.e., terms of E of not less than square. The second harmonic generation (SHG) expressed by the second term and the third harmonic generation (THG) expressed by the third term are well known as the effects of frequency conversion. The third term also excerts effects of light intensity dependent optical constants, such as effect of optically induced refractive-index change, nonlinear absorption effect, the second-order electric optical (Kerr) effect and four-wave parametric mixing (phase-conjugated-wave generation) effect, any one of effects is important in the applied optics field.
The effect of optically induced refractive-index change is an effect by which the refractive index (n) of a substance is changed depending on the intensity of the impinging light in accordance with the equation of EQU n=n.sub.0 +n.sub.2 I
(wherein n.sub.0 is a constant, n.sub.2 is nonlinear refractive index coefficient, and I is intensity of impinging light). By making an element from the substance showing this effect and by combining the element with other optical elements such as optical resonator, polarizer and reflecting mirror, devices such as optical bistable device and phase-conjugated-wave generator, which are important in optical data processing systems and optical communication systems, can be constituted.
As an example of such devices, optical bistable device will now be explained. An optical medium having a nonlinear refractive index is sandwiched between a pair of vacuum-deposited dielectric mirrors with a reflectance of about 90%. An input light impinges on one of the mirrors and exits from another mirror as an output light after repeatedly passing through the substance. If the wave-length of the input light is slightly changed to satisfy the resonance condition, the intensity of the output light (Pt) changes in response with the intensity of the input light (Pi) as shown in FIGS. 1 (b) and (c). The principle of this phenomenon is detailed in Appl. Phys. Lett. vol 35, p. 451 (1976). The responses shown in FIGS. 1 (b) and 1 (c) correspond to optical limiting and optically bistable responses, respectively, and they may be applied to reshaping of the waveform of input light pulse, all-optical switch, optical memory and optical logic operation behavior in optical communication systems and optical data processing systems.
As the nonlinear optical element, the following three characteristics are extremely important:
1) Width of the wave-length range of input light which may be employed, PA0 2) Minimum intensity of the input light which is required for the operation, and PA0 3) the response time required for following the change of the intensity of the light signal.
As a conventional nonlinear optical element, super lattice semiconductor thin films crystal produced by alternately and repeatedly growing GaAs and GaAlAs are used. The operation of this element is based on the principle that the excitons are excited in the crystal by the absorbed light so that the refractive index shows light intensity-dependence. Although the element is excellent in that the minimum intensity of the input light required for the operation is as low as 5.times.10.sup.4 W/cm.sup.2, it has drawbacks in that the wave-length range of the input light which may be employed is restricted to a very narrow range in the vicinity of the exciton absorption spectrum, and in that the response time is determined by the exciton lifetime, so that it cannot be used for the optical signal processing which requires a response time of shorter than 3.times.10.sup.-8 second.
In another conventional optical bistable device, a glass cell in which optical Kerr liquid, carbon disulfide (CS.sub.2) is filled is employed, and a pair of external mirrors are employed in place of the vapor-deposited mirrors. With this conventional device, since the operation principle is based on the fact that the refractive index shows light intensity-dependence by the rotational arrangement of the molecules in response to the light-electric field (molecular-rotational orientation Kerr effect), although the employable wave-length range of the input light is as wide as from visible light to near infrared light, the device has drawbacks in that the minimum light intensity required for the operation is as high as 10.sup.7 -10.sup.8 W/cm.sup.2 or the element must be made large because the efficiency of molecular-rotational orientation Kerr effect of the carbon disulfide is small, and in that the response time is determined by the rotational relaxing time of the molecules, so that it cannot be used for the optical signal processing which requires a response time of shorter than 10.sup.-11 -10.sup.-12 second.
In order that the nonlinear optical element may practically be used, it is desired that semiconductor laser having a wave-length range of 0.65-2.3 .mu.m and an effective output power of 30 mW can be used as a light source.
As is apparent from the above description, the performance of the nonlinear optical element is substantially entirely determined by the characteristics of the nonlinear optical substance. Thus, nonlinear optical materials in which 1) the employable wave-length range of input light employed is wide, 2) the minium intensity of the input light required for the operation is low, and 3) the response time required for following the change of the intensity of the light signal is short, are now intensively looked for.
Recently, organic compounds which have a .pi.-conjugated system such as aromatic ring, double bond and triple bond, especially polydiacetylene-based compounds are drawing attention. For example, poly (2,4-hexadiyn-1,6-diol bis(p-toluenesulfonate) (PTS for short) has a third-order nonlinear optical susceptibility X.sup.(3) of as high as 1.times.10.sup.-10 esu in our measurement at 1.90 .mu.m of resonant wavelength region (when converted into nonlinear refractive-index coefficient n.sub.2, this corresponds to n.sub.2 =2.times.10.sup.-12 (W/cm.sup.2).sup.-1)), while G. Sauteret reported the values 8.5.times.10.sup.-10 esu at 1.89 .mu.m of resonant wavelength region and 1.8.times.10.sup.-10 esu at 2.62 .mu.m of non-resonant wavelength region (Phys. Rev. Letters, Vol 36, pp. 16 (1976). Anyway, X.sup.(3) of PTS is larger than that of carbon disulfide at least by two orders of magnitude. Further, since the nonlinear optical effect is entirely based on the .pi.-polarization in a molecule, and not on the absorption where the light and the molecules or crystal lattices interact, the response time for following the intensity change of the light signal is as short as 10.sup.-14 seconds. Still further, the employable wave-length of the input light ranges as wide as from 0.65 .mu.m to 2.0 .mu.m.
The polydiacetylene compounds represented by PTS are produced by polymerizing diacetylene monomers in crystal state. However, after the polymerization, the resultant polydiacetylene compounds are insoluble and infusible, so that they are hard to process. That is, it is difficult to obtain a polydiacctylene-based nonlinear optical element which has a desired shape or desired surface smoothness. Thus, nonlinear optical element comprising the PTS has not yet been practically used.
Further, from a practical point of view, it is desired to improve the operation performance of the element, to reduce the output power of the light source and to make the optical element in small size, by employing a nonlinear optical material with nonlinear optical effects greater than those of PTS.